Housing units comprise houses, apartments, groups of rooms, or single rooms, which are occupied, or vacant but intended for occupancy, as separate living quarters. Specifically, there is a housing unit when the occupants live and eat separately from any other persons in the structure and there is either (1) direct access to the unit from the outside or through a common hall, or (2) in 1960, a kitchen or cooking equipment for the occupants exclusive use: in 1970, complete kitchen facilities for the occupants exclusive use.
Structures intended primarily for business or other non-residential use may contain housing units; for example, the living quarters of a merchant in back of his shop. Separate living quarters occupied by staff personnel (but not inmates) in institutions which meet the definitional criteria constitute housing units; as do separate living quarters of supervisory staff in dormitories, nursing homes, etc. Any separate living quarters, which meet the above criteria, in rooming or boarding houses are classified as housing units, as are entire rooming or boarding houses where there are four or fewer roomers unrelated to the person in charge. Trailers, tents, boats, railroad cars, hotel and motels occupied by usual residents which meet the definitional criteria constitute housing units; as do vacant rooms or suites in hotels where 75 percent or more of the accommodations are occupied by usual residents.

Housing units are classified by the number of units (including occupied and vacant , excluding business units or group quarters) in the structure in which they are located, as indicated below. Data are tabulated only in terms of housing units. Except for one-family houses (detached and attached), there is no information regarding number of structures. In 1960, determination of units in structure was by enumerator observation or, by inquiring of the landlord, the janitor, etc.; in 1970, by respondent replies to a question on whether this is a building for one family, 2 families, etc. (Categories which respondents could specify are indicated below.)

A structure is defined as a separate building that either has open space on all four sides (detached), or is separated by dividing walls that extend from ground to roof (attached). Tabulations of this and other structural characteristics are in terms of number of housing units rather than number of structures.

1-unit structures detached from any other house, i.e., with open space on all four sides. Such structures are considered detached even if they have an adjoining private garage or contain a business unit.

Occupied trailers or mobile homes are shown separately from other 1-unit detached structures in some tabulations. In 1960, trailers were further classified as mobile (resting on wheels or on a temporary foundation, such as blocks or posts), or on permanent foundation (mounted on a regular foundation of brick or concrete, etc.). In 1970, this breakdown was omitted.

Structures containing 2 or more housing units; further broken down as 2-units, 3 or 4-units, 5 to 9-units, 10 to 19, 20 to 49, 50 or more units.
In 1970, to reflect the wording on the questionnaire, tabulations of units in structure are sometimes in terms of buildings for two families, 3-4 families, etc.